1. Field of the Invention
This invention relates to a process for preparation of a current limiting composition and is in particular a process for preparation of an improved positive temperature coefficient (PTC) thermosetting epoxy polymer composition which has a low resistivity and temperature rise when used as a circuit component carrying normal current. Due to the present process for preparation of the epoxy when the current increases due to short circuit or overload, the composition temperature increases and changes to a high resistance state which limits the current to a safe value until the current is interrupted. When the current is interrupted, the thermosetting material cools and reverts back to its original low resistance state. The improved product made by this process has a specific application as a resettable fuse or current limiter in motor controls and switch gear equipment.
2. Background Information
Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under normal operating conditions of the circuit, but are xe2x80x9ctripped,xe2x80x9d or converted to a high resistance state when exposed to excessive current or temperature. When the high resistance is caused by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature which switches it to the high resistance state. The increase in resistance is accompanied by an expansion of the PTC element.
Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them are well known as set forth, for example, in U.S. Pat. Nos. 5,098,940, 5,174,924 and 5,195,013. In recent years, they have been widely used for protecting motors, solenoids, telephone lines and batteries. For instance, PTC devices have been utilized as current limiters connected in series with the separable contacts of a current interrupter such as a circuit breaker. The PTC device prevents the short circuit current from reaching a level at which the circuit breaker becomes incapable of interrupting this current due to the inability to extinguish the arc struck between the separated contacts. Various polyethylene PITC compositions and electrical devices incorporating them are shown for example in U.S. Pat. Nos. 5,049,850; 5,195,013; 4,724,417; 4,545,926; 4,685,025; 4,647,894; 4,857,880; 4,910,389; 4,774,024; and 4,775,778.
Articles have also been published in the field such as xe2x80x9cThermistors For Overcurrent Protection,xe2x80x9d Machine Design, Dec. 10, 1981, pp. 161-165; xe2x80x9cConductive Polymers,xe2x80x9d Machine Design, Oct. 22, 1992, p. 161; xe2x80x9cCircuit Protectors,xe2x80x9d Machine Design, Dec. 12, 1991, p. 82; and xe2x80x9cConductive Polymers,xe2x80x9d Design News, Nov. 9, 1992, p. 99.
A process for making an improved PTC conductive polymeric composition has been provided which uses a thermosetting epoxy resin combined with conductive particles of carbon black and/or metal as a current limiting composition which has many technical advantages over the current state of the art. These advantages include: (a) improved stability of epoxy resin over polyethylene resin; (b) less degradation and xe2x80x9csheddingxe2x80x9d of carbon than seen with the polyethylene PTC devices; (c) a more compact polymer system in which conductive additives and epoxy chains are chemically locked and intertwined into a stable polymer matrix; (d) improved high voltage arc and track resistance and diminished partial discharge characteristics; and (e) lack of plasticizing effect caused by thermoplastic additives which causes creeping or a ratcheting effect when thermoplastic separates from the epoxy matrix with each thermal cycle.
The PTC current limiting composition comprises a chemically cross-linked epoxy thermoset resin polymer and conductive additive particles dispersed in the resin. The thermosetting epoxy resin comprises more than one 1,2 epoxy groups per molecule and an acid anhydride selected from the group consisting of monofunctional anhydrides effective as a curing agent for the epoxy resin. The thermosetting epoxy resin further comprises an epoxy reactive diluent. The conductive additive particles are selected from the group consisting of at least one of carbon black, nickel fiber, nickel flake, nickel beads and copper flake. The thermosetting epoxy resin is selected from the group consisting of bisphenol A epoxy resins, novolac epoxy resins and mixtures thereof and the epoxy reactive diluent is selected from the group consisting of phenyl glycidyl ether, butyl glycidyl ether, alkyl glycidyl ethers containing about 5 to 12 carbons, vinyl cyclohexene dioxide, endodicyclopentadiene dioxide, octylene oxide and neopenylglycol diglycidyl ether. The acid anhydride is selected from the group consisting of hexahydrophthalic anhydride, 1-methyl hexahydrophthalic anhydride, tetrahydrophthalic anhydride, 1-methyl tetrahydrophthalic anhydride, phthalic anhydride, polyazelaic polyanhydride, benzophenone tetracarboxylic acid dianhydride and mixtures thereof. The bisphenol A epoxy resin has an epoxy equivalent weight of from about 130 to 1200, the novolac epoxy resin has an epoxy equivalent weight of from 100 to 500 and the composition is cured. Reduction of porosity occurs by heating to between about 40xc2x0 C. and 60xc2x0 C. and drawing a vacuum of less than or equal to 10 m torr on the mixture. The temperature is carefully chosen so that the resin stays liquid during the vacuum cycle. Gelation (initial hardening) occurs at a temperature of about 130xc2x0 C. to 140xc2x0 C. in about 2 to 4 hours. The final cure takes about 13 to 18 hours at about 140xc2x0 C. to 160xc2x0 C. At the initial mixing, co-accelerators are added to give a faster cure at lower temperatures. The co-accelerators preferably comprise chromium acetyl acetonate accelerator and 1-methyl imidazole. An effective amount of alumina trihydrate is added to prevent dielectric breakdown, arcing and carbon tracking under high voltage conditions.
An improved current limiting composition comprises a chemical cross-linked cured epoxy thermosetting resin having a plurality of conductive additive particles dispersed therein and is made by the process of mixing an epoxy thermosetting resin containing more than one 1,2 epoxy groups per molecule with an acid anhydride curing agent, an epoxy reactive diluent, co-accelerators, and conductive additive particles to form a liquid mixture, heating the liquid mixture at a first temperature range below gelation temperature while drawing a vacuum to drive off volatiles and gases, heating at a second temperature range to promote gelation, heating at a third temperature range to effect a final cure using a first accelerator which does not become effective until the gelation temperature range and using a second accelerator which does not become effective until final cure temperature range.
An effective amount of said alumina trihydrate is added to the liquid mixture to prevent dielectric breakdown, arcing and carbon tracking under high voltage conditions in the current limiting compositions. The first co-accelerator is 1-methyl imidazole. The second co-accelerator is chromium acetylacetonate.
The first temperature range is about 40xc2x0 C. to 60xc2x0 C. A vacuum pump pulls a vacuum of about xe2x89xa610 m torr on the liquid mixture. The second temperature range is about 130xc2x0 C. to 140xc2x0 C. The third temperature range is about 140xc2x0 C. to 160xc2x0 C. Gelation takes about 2 to 4 hours in the second temperature range and final cure takes about 13 to 18 hours in the third temperature range.
The epoxy resin is a diglycidyl ether of bisphenol A, the epoxy reactive diluent is preferably a diglycidyl ether of neopentyl glycol, the acid anhydride hardener is preferably 1-methyl tetrahydrophthalic anhydride and the conductive additive particles are selected from the group consisting carbon black, nickel fiber, nickel flake, nickel beads and copper flake.
A process for making an improved current limiting composition comprises mixing an epoxy thermosetting resin containing more than one 1,2 epoxy groups, referred to as xe2x80x9cepoxy groupsxe2x80x9d herein, per molecule with an acid anhydride curing agent, an epoxy reactive diluent, co-accelerators, and conductive additive particles to form a liquid mixture, heating the liquid mixture at a first temperature range below gelation temperature while drawing a vacuum to drive off volatiles and gases and continued heating at a second temperature range to promote gelation. A first co-accelerator is used which does not become effective until the gelation temperature range. The mixture is then heated at a third temperature range to effect a final cure, using a second co-accelerator which does not become effective until final cure temperature range.